It depend perhaps on the mode of operation. If you work in batch it's too easy, the ash is just what remains in the bed after the process. When you want to remove ash during continuous processing, you must be aware that bed is a mixture of ash and fresh or partially gasified coal. When you substract material from the bed, you also remove a certain amount of feed since a fluidized bed is, al least in theory, a ideally mixed reactor. It may be different when you have contents that differ a lot in density, then there may be segregation. Is the ash lighter than the coal or reverse? In my thesis (Delft University, 1985) I do have some information on segregation but I can not see whether it is relevant for your system. 

Thank you for the comment. Let me clarify on that.

The system I am working is fluidised bed gasification for coffee husk, continuous type. During operation, there are three solids; bed ash, partially gasified biomass and the sand granules. How the density difference methods helps me here?

Besides the syngas, there are other undesirable materials: particulate materials (which you know, i.e. fly ash & solid particle which may include coke), tars, and a number of volatilized impurities. The cyclone removes the particulate solids & an electrostatic precipitator helps in their deposition down through a line into a collector while another line passes into fabric & ceramic filters to get rid of the other impurities. Both lines allow for recycling & utilization of the wastes. The final outlet line is assumed to contain syngas only.

I presume that the most dense material concentrates in the lower part of you fluidized bed. That is probably the sand. But you would not like to remove that? It is the ash you want to get rid of. Perhaps you must trust on the fluidized bed to grind it up during the operation and then, it will be entrained with the gas and can be removed via the cyclone bottoms. Take care, also the fly-ash will contain a percentage of ungasified feed material. You can measure that by (simple?) TGA or combustion of the removed ash and correct conversion data for that. 

Considering small scale application an entrainment of small or light ash particles seems the easiest way. Then gas should be cooled down to condensate volatilized impurities at the surface of ash fines (quite typical behaviour of nucleation). Cyclone can be used in next step. However in case of large content of tar and volatilities it can be plugged quite easily. In such a case impurities (particles/droplets) can be removed during gas flow through the water spray and particle-drop or drop-drop collisions. Produced slurry can be separated by filtration. Water can be separated from liquid volatilities then returned to the system and then lquid volatilities and solid/tar particles can be burned.

Such solution seems reasonable in relatively small scale plant. In bigger ones electrostatic precipitators are more profitable.

there are two kinds of bed ash removal facilities available for fluidized bed gasigier or fluidized bed combustor. One is so called fluidized bed ash cooler that can both cool down the bed ash and separate the lighter or fine char particles from heavy sand and return the char back to the reactor. Another is a simple rotary water jacketed ash cooler that only cool down all drain bad ashes and remove them out. The second type ash cooler could be found in Chinese market.

Removal of bed ash, particulate material and tar via cyclone requires min operational velocity of fluidized bed. It is in range of 2-4 m/s. In gasifier if we can keep velocity less all ash and tar will deposit on bed material itself provided its particle size in range of 100-250 micrometer. You can very easily drain bed via drain pipes in the bed. Suitable Air Lock and cooling mechanism need to be devised for hot bed material. Bed can be continously filled with fuel so that its level remains constant.

First you should check the terminal velocity of char, ash and raw material. And during the operation you have to adjust the velocity greater than to terminal velocity of char and less than to terminal velocity of raw material. For example raw rice husk has a terminal velocity of 1.2 m/s and rice husk char has a terminal velocity of 0.4 m/s. So during the gasification operation we kept the velocity of air at 0.6-0.7 m/s. You can refer my papers.

I agree but consider the case of oxygen cumbustion instead of air, the bed operating temp is near 1700degrees or 1973K. How can we remove ash at such high temperatures ??. Intention is to get nitrogen free flue gas and fuel has quality to reach these temp to be used in gas turbines. Outbed char removal is very difficult here.

In fluidized bed gasifiers char is removed though cyclone separators. If your cyclone is not removing 100% chars then you have to add another cyclone separator. Also you have to quantify the char material which is to be discharge from the cyclone separator. On the base of char discharged, you have to maintain the fuel feeding rate.